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It has been determined that the capacity of a mill is related to its diameter by a factor of 3.
This condition obtains only if the total diameter is utilized, i.e. the charge is lifted to a point that provides maximum drop height for the grinding bodies.
This point is located at 11 o’clock on a circle representing a cross-section of the mill.
The location of the point where the grinding bodies impact the mill lining is just as important as the location of the point where they start to fall. The optimial drop would be from 12 to 6’ O clock, but this is of course impossible to achieve. Lining design is thus extremely important.
A liner that includes lifter bars offers unlimited potential for identifying the optimal solution. Steel liners do not usually provide the required lifting action. A composition of Figs. D with Figs. A shows that mill speed affects the drop height. This must naturally be taken into account when the lining is designed.
However, the shape and height of the lifter bars are the most important factors.
From the standpoint of liner economy, the lifter bars should be as high as possible in order to provide maximum service life, although the required grinding performance is a constraint on lifter bar height.
The lining profile in Fig. G75 illustrates the situation in a newly relined mill.
The balls start to drop at a point above 11 o’clock and impact the lining near 5 o’clock. After a short period of operation, the lifter bars become rounded off, so that the charge behaves as shown in Fig. A75. As the lining becomes more worn, the cascade is shaped as in Fig. E75.
The bars become rounded off after rounded off after roughly 10% of service life has elapsed. The second stage of wear (Fig. A75) accounts for the by far the greater part of service life.
Thus the living should be designed so as to provide an optimal profile (Fig. A75) for the major portion (the second stage) of its service life.
As service life of the lining draws to a close, a higher charge level can compensate for the increased wear. A greater quantity of grinding bodies has the effect of increasing the lifting height and also increased the degree of contact within the charge.
If a lining were installed with a original design as in Fig. A75, it would be worn down as shown in Fig. F75 by the time it entered the second and longest stage of its service life.
An important factor related to drop height is the decrease in charge pressure as lifting height is increased.
In very general terms, a high charge pressure is needed for fine grinding, while a high drop height is needed to obtain enough impact force for primary grinding.
These two factors must therefore be balanced, and different profiles must be fitted for different types of grinding.
A profile with sharply beveled lifting bars as in Figs. F is appropriated for fine grindning. The profile shown in Figs. B is best for primary grinding.
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